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    <title>Document</title>
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<body>
    <script>
      var i=99;

      //.......

      for(let i=0;i<5;i++){
          console.log(i);
      }

      //输出i的值
      console.log(i);//5,for循环的i污染了外部的i=99,所以要换一个变量定义方式 let const
    </script>
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